Giving a public IP address to one device

I have a very basic setup where my Asus WL-500W acts as a router for all my devices and connects them to the Internet. Basically everything just works with the default Tomato (Shibby) settings.

However now I would like to assign another public IP address to one of my devices which connects via ethernet to the Asus. How to accomplish this? I would still like to connect to the device from the internal LAN so it should also have a private IP address. My ISP provides me with at least two public IPv4 addresses (DHCP).